The present invention relates to a nonaqueous electrolyte secondary cell, and more particularly to a carbon material for negative electrode of a lithium ion secondary cell.
As a nonaqueous electrolyte secondary cell, a so-called lithium secondary cell has been hitherto studied in the interest of higher energy density by means of higher voltage and larger capacity, in which metal lithium is used as a negative electrode active material, whereas oxide, sulfide, selenide or other chalcogen compounds of transition metal, such as manganese dioxide, molybdenum disulfide or titanium selenide is used as a positive electrode active material, and organic electrolyte made of organic solvent solution of lithium salt is used as a nonaqueous electrolyte. In this lithium secondary cell, however, while an interlayer compound which exhibits relatively good charge and discharge characteristics may be selected as a positive electrode active material, the charge and discharge characteristics of metal lithium for negative electrode are not particularly excellent. Thus, the cycle life for repeating charge and discharge can hardly be extended, and moreover, there is a danger that heat generation may be caused by internal short-circuit, presenting a problem in safety. More specifically, the metal lithium in the negative electrode active material elutes into the organic electrolyte as lithium ions by discharge. When the eluted lithium ions precipitate on the surface of the negative electrode as metal lithium by charge, not all of them precipitate smoothly as in the initial state, but some precipitate as active metal crystals in the form of dendrite or moss. The active metal crystals decompose the organic solvent in the electrolyte, while the surface of the metal crystals is covered with a passive film to be inactivated, hardly contributing to discharge. As a result, as charge and discharge cycles are repeated, the negative electrode capacity declines, wherefore the negative electrode capacity had to be set extremely larger than that of the positive electrode when fabricating a cell. Besides, the active dendritic metal lithium crystals may pierce through the separator and contact with the positive electrode, possibly causing internal short-circuit. By internal short-circuit, the cell may generate heat.
Accordingly, the so-called lithium secondary cell, which uses a carbon material that is capable of repeating reversibly intercalation and deintercalation by charge and discharge as the negative electrode material, has been proposed, is now being intensively researched and developed, and is already put in actual use. In this lithium secondary cell, so far as it is not overcharged, active dendritic metal lithium crystals do not precipitate on the negative electrode surface when the cell is charging up and discharging, and enhancement of safety is much expected. Moreover, since this battery is extremely superior in high rate charge and discharge characteristics and cycle life to the lithium secondary cell using metal lithium in the negative electrode active material, the demand for this battery is growing rapidly in recent years.
As the positive electrode active material for lithium ion secondary cell of 4V class, a composite oxide of lithium and transition metal, such as LiCoO2, LiNiO2, LiMnO2, and LiMn2O4, corresponding to the discharge state is being employed or considered. As the electrolyte, similarly as in the lithium secondary cell, a nonaqueous electrolyte such as organic electrolyte and polymer solid electrolyte is used.
When graphite is used in the negative electrode material, the theoretical value of capacity per 1 g of carbon by reference to C6Li of interlayer compound produced by intercalation of lithium ion is 372 mAh. Therefore, among various carbon materials, the one which helps realize a specific capacity close to this theoretical value, as well as causes the capacity per unit volume, i.e., capacity density (mAh/cc) to be as high as possible, should be selected for the negative electrode that is put in practical use.
Among various carbon materials, in the hardly graphitized carbon generally known as hard carbon, materials which exhibit a specific capacity exceeding the above mentioned theoretical value (372 mAh/g) are discovered and are being investigated. However, since the hardly graphitized amorphous carbon is small in true specific gravity and is bulky, it is substantially difficult to increase the capacity density per unit volume of the negative electrode. Furthermore, there still remain many problems, for example, the negative electrode potential after charge is not so base as to be close to the metal lithium potential, and flatness of discharge potential is inferior.
By contrast, when natural graphite or artificial graphite powder which is high in crystallinity is used in the negative electrode, the potential after charge is close to the metal lithium potential, and the flatness of discharge potential is excellent, whereby the charge and discharge characteristics are enhanced as a practical battery, and thus the graphite powder is recently becoming the mainstream of negative electrode material.
However, when the mean particle size of the graphite powder for negative electrode of a lithium ion secondary cell is large, the charge and discharge characteristics at high rate and discharge characteristic at low temperature tend to be inferior. Accordingly, by reducing the mean particle size of the powder, the high rate charge and discharge characteristics and low temperature discharge characteristic are enhanced, but if the mean particle size is made too small, the specific surface area of the powder becomes too large, as a result of which there is a problem of increased irreversible capacity, in which the lithium inserted by first charge in the powder cannot contribute to discharge after the first cycle. This phenomenon is not only a fatal demerit for enhancement of energy density, but also causes the solvent in the organic electrolyte to be decomposed in case the battery is left at a high temperature exceeding 100xc2x0 C., which may lead to self-discharge as well as an electrolyte leak accident due to raise in the cell internal pressure, thereby lowering the reliability of the battery.
It is hence easily understood that the appropriate specific surface area and mean particle size are essential for the graphite powder for negative electrode. An invention proposed from such viewpoint is, for example, Japanese Laid-open Patent No. 6-295725, which uses graphite powder of which specific surface area by BET method is 1 to 10 m2/g, mean particle size is 10 to 30 microns, and at least one of the content of powder with a particle size of 10 microns or less and the content of powder with a particle size of 30 microns or more is 10% or less. Further, in Japanese Laid-open Patent No. 7-134988, the usage of spherical graphite powder is disclosed, which is obtained by graphitizing meso-carbon micro-beads formed by heating petroleum pitch at a low temperature and of which plane interval (d002) of (002) plane by wide angle X-ray diffraction method is 3.36 to 3.40 angstroms, and specific surface area by BET method is 0.7 to 5.0 m2/g.
These inventions were not only extremely effective for enhancement of high rate charge and discharge characteristics and discharge characteristic at low temperature of the lithium ion secondary cell, but also effective for decreasing the irreversible capacity determined in the initial phase of cycle, which was a fatal problem to be solved. However, such problems are still left that storage property and reliability when left at a high temperature are not sufficiently achieved, and the specific capacity (mAh/g) and capacity density (mAh/cc) of the negative electrode are not satisfactory.
It is thus an object of the invention to further improve the reliability and high energy density of a lithium secondary cell.
To solve the aforesaid problems of the lithium ion secondary cell, according to the present invention, flaky or lumpy graphite particles of high purity (fixed carbon content 98% or more) and of high crystallinity with a mean particle size of 20 microns or more and a mean thickness of 15 microns or more are dispersed in liquid or gas; this liquid or gas is pressurized and discharged spirally from the nozzle, thereby pulverizing the graphite particles; and they are sifted, by which the tapping density, X-ray diffraction peak intensity ratio of (110)/(004) by wide angle X-ray diffraction method, and particle shape of the graphite powder are controlled, whereby a nonaqueous electrolyte secondary cell of high specific capacity is realized, wherein the irreversible capacity noted in the initial cycle is kept as small as possible, the storage property and reliability of the battery when left at a high temperature are enhanced, and excellent high rate discharge characteristic and discharge characteristic at a low temperature are achieved.